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The streak plate technique, using solid media in a Petri plate, is an important isolation technique in microbiology. When done properly, streak plating makes it possible to isolate individual species of bacteria from a mixed culture.

Which of the following observations would suggest that a plate was inoculated with a pure (axenic) culture?
a) Isolated colonies are all white in color and about the same size.
b) Bacterial growth is apparent along the streaks connecting each quadrant.
c) Bacterial growth is apparent in all four quadrants.
d) Isolated colonies are all white in color, but some colonies are noticeably larger than others.

a) Isolated colonies are all white in color and about the same size.
Bacterial species tend to produce distinctive-looking colonies. If all colonies on the plate appear identical, then it's likely the inoculum was pure.

Imagine that you forgot to flame the loop before streaking the inoculum from the first quadrant into the second quadrant. What is the most likely consequence of this error?
a) Too much bacterial growth outside the first quadrant.
b) Contamination of the broth culture.
c) Too little bacterial growth outside the first quadrant.
d) Contamination of the Petri plate culture.

a) Too much bacterial growth outside the first quadrant.
Proper streak plating technique deposits progressively smaller amounts of bacteria in each quadrant. This is done by having the loop pick up only a small amount of bacteria from the previous quadrant's streaks. Flaming the loop between streaks ensures that the loop starts clean and that only this small amount of bacteria is used to inoculate the next quadrant.

A student INCORRECTLY streaks the plate according to the diagram below, creating streaks in the order: a, b, c, d. Which of the following is a streaking error shown in the illustration?
a) Streak d does NOT draw inoculum from streak c.
b) Streak c does NOT draw inoculum from streak a.
c) Streak c goes outside quadrant 3.
d) Streak b draws inoculum from streak a.

a) Streak d does NOT draw inoculum from streak c.
For a plate to be streaked correctly, streak d must draw inoculum from streak c. In this example, streak d draws inoculum from streak a instead. This will draw too much bacteria into quadrant 4 and produce few, if any, isolated colonies.

Which of the following images shows how to properly open a Petri plate to streak the agar?
a) lifting the lid only enough to get the loop inside.
b) lifting the lid completely to the left side.
c) removing the lid and placing it on your work bench.
d) lifting the lid and allowing it to rest on top of the inoculating loop.

a) lifting the lid only enough to get the loop inside.
Lifting the lid of the Petri plate only enough to get the loop inside keeps the agar protected. Most of the lid remains right above the agar, and this prevents airborne microbes from falling into your culture.

Where should you label your Petri plate with information about the culture?
a) On the inside of the lid.
b) On the top of the lid.
c) On the bottom of the plate.
d) On the side of the bottom (agar-containing) portion of the plate.

c) On the bottom of the plate.
All labeling should be done on the bottom portion of the agar plate. This ensures that the written information stays with your culture, even if the lid gets accidentally exchanged with another plate.

Imagine you're streaking a plate. You've just finished the first streak (streak a), closed the lid, and rotated the plate. List the next steps of the procedure in their correct order.

A student streaks a plate using inoculum from a broth culture tube. The culture tube was labeled "Escherichia coli," indicating that the culture was composed of a single bacterial species. Which of the following best explains the unexpected plate result shown here after incubation?
a) The plate was exposed to airborne contaminants.
b) The loop wasn't flamed between quadrant streaks.
c) The plate was not rotated properly during the inoculation process.
d) The culture in the tube was contaminated.

d) The culture in the tube was contaminated.
E. coli colonies should all have the same color, size, shape, and margins. However, there are colonies on this plate that have different color, size, and shapes. This means that another bacterial species is present—a contaminant. Notice, too, that both colonies are growing along the streak lines. This suggests that both species came from the broth tube. Airborne contaminants introduced during streaking would, in contrast, be more randomly scattered about the plate.

If a correctly streaked plate were INCORRECTLY incubated right side up (instead of upside down), which of the following plate results would you most likely see?
a)
b)
c)
d)

b)
During incubation, water tends to condense in the lid of Petri plates. If the plate is incubated in a right-side-up position, this condensation will "rain" onto the surface of the agar. The water on the agar will create a soupy mess of bacteria instead of isolated colonies. Incubating the plate upside down keeps water condensation in the lid and away from your culture.

A student performs a streak plate using inoculum from a broth culture tube. The culture tube was labeled "Escherichia coli," indicating that the culture was composed of a single bacterial species. Which of the following best explains this unexpected plate result seen after incubation?
a) The plate was exposed to airborne contaminants.
b) The culture in the tube was contaminated.
c) The plate wasn't incubated in an inverted position.
d) The loop wasn't flamed between quadrant streaks.

a) The plate was exposed to airborne contaminants.
When a plate is exposed to airborne contaminants, the contaminating microbes settle all over the plate. After incubation, the colonies from these contaminants will be all over the plate also, not just following the streak lines.

A student performs a streak plate using inoculum from a broth culture tube. The culture tube was labeled "Escherichia coli," indicating that the culture was composed of a single bacterial species. Which of the following best explains this unexpected plate result seen after incubation?
a) The loop wasn't flamed between quadrant streaks.
b) The culture in the tube was contaminated.
c) The plate was contaminated during streaking.
d) The plate wasn't properly rotated between streaks.

a) The loop wasn't flamed between quadrant streaks.
You want to spread only a tiny fraction of the bacteria in one quadrant to the next, so it's important to flame and cool the loop between each streaking step. This ensures thinning of the bacteria with each streak, which ultimately produces isolated colonies after incubation.

A streak plate was prepared from an unknown culture. Which of the following best explains this plate result seen after incubation?
a) The unknown culture contains two species of bacteria.
b) The plate was contaminated during the streak plate process.
c) The loop wasn't flamed before streaking into quadrant 4.
d) The plate was not incubated in an inverted position.

a) The unknown culture contains two species of bacteria.
There are multiple distinct colonies along the streak lines consistently in each quadrant. This suggests that more than one microbe was present in the initial inoculum.